Austenite Recrystallization-Precipitation Interaction in Niobium Microalloyed Steels

A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at lower temperatures by solute drag or by precipitation pinning. In this study, the recrystallization behavior of four Nb-microalloyed model alloys which were designed to show either extensive or almost no precipitation, was compared by multi-hit torsion tests and double hit compression tests. A good consistency between the different types of tests was found and the results were verified by optical micrographs. Further, by construction of softening-time-temperature diagrams the recrystallization behavior was linked to the precipitation state of the material which was investigated by thermodynamical equilibrium calculations and by experimental observations from TEM-EDX, Inductively Coupled Plasma Mass Spectroscopy and X-ray Diffraction. Quantitative agreement between the experimental measurements and the calculations for precipitated mass fraction and precipitate composition as a function of temperature and steel composition is demonstrated.